Just spotted 2 other cheap events weekends at Stowford:
VW Rally (open to all) May Day Bank Holiday 3 nights 29 April - 2 May pitch for £40 up to 6 people plus extra nights at £5 per night. No current clashes with other Fun events that I have spotted so far.
Dog Show 3-6 June 3 nights pitch for £50 with up to 6 people, EHU, dogs & awning incl.
See http://stowford.co.uk/caravanning-camping-in-devon/whats-on-at-stowford/